quick question on resistors and leds

[donal]

using this guide http://www.e-cigarette-forum.com/fo...asic-wire-diagram-flashlight-mod-diagram.html what value is the resistor between the led and ground sopose to be iv read in another one that youre sopose to use 500 omes seems like over kill without quoting omes law (far to much math for my likeing) what value do i need and buy the looks of it would i need one at all if i had no led built in also im using 5 volt leds . i also have evry thing else for a 5v olt mod using 2 3.7 batterys what resistor values will i need in this set up also any other usfull values i should pick up while im at it as the electronic shop as it is a good 10 miles away from me

 

[donal]

just realesed the 500 omes is not that much

 

[jrm850]

(Supply volts - LED forward Volts)/amps

example would be 5volt supply -Forward voltage of 2 volts = 3 volts. 3volts divided by 20milliamps (.020amps)= 150ohm resisitor.

or you can just use one of the online calculators like this: LED Resistor Calculator

 

[Str8V8ping]

Heres a good led calculator for you .The resistor you ise will be different with every LED since all LEDs run at different FV and current specs . With any ecig mods you should be able to use a 1/4w resistor as ecigs run on a low supply voltages .

LED series parallel array wizard